Li-Jing Cheng

Ionic current rectification, breakdown, and switching in heterogeneous oxide nanofluidic devices.

We investigate several ion transport behaviors in sub-20 nm nanofluidic channels consisting of heterogeneous oxide materials. By utilizing distinct isoelectric points of SiO2 and Al2O3 surfaces and photolithography to define the charge distribution, nanofluidic channels containing positively and negatively charged surfaces are created to form an abrupt junction. This method provides much more robust surface charges than previous approaches by surface chemical treatment. The fabricated nanofluidic diodes exhibit high rectification of ion current and achieve record-high rectification factors (ratio of forward current to reverse current) of over 300. The current-voltage property of the device follows the theoretical model quantitatively, except that at low ion concentrations the forward current degrades and the reverse current is greater than theoretical prediction, which can be attributed to access resistance and breakdown of water molecules. The breakdown effect characterized by a negative conductance followed by a rapid increase of current is observed in a double junction diode. The occurrence of the breakdown is found to be enhanced by the abruptness of the junction between the heterogeneous nanochannels. Finally, we demonstrate ionic switching in a three-terminal nanofluidic triode in which the ionic flow can be electrically regulated between different channel branches. The study provides insight into the ion transport behavior in nanofluidic devices containing heterogeneous surfaces.

Microscale pH regulation by splitting water

We present a simple, flexible approach for pH regulation in micro-chambers by injecting controllable amounts of protons and hydroxide ions via field-enhanced dissociation of water molecules. Under a DC voltage bias, the polymeric bipolar membranes integrated in microfluidics devices generate and separate H(+) and OH(-) ions without gas production or contaminant generation resulting from electron-transfer reactions. Robust local on-chip pH and pH gradients are sustained with no need of additional acidic∕basic solutions that dilute analyte concentrations. The method could provide a better strategy for pH control in microfluidics.

Biomolecular motor-driven molecular sorter

We have developed a novel, microfabricated, stand-alone microfluidic device that can efficiently sort and concentrate (bio-)analyte molecules by using kinesin motors and microtubules as a chemo-mechanical transduction machine. The device removes hundreds of targeted molecules per second from an analyte stream by translocating functionalized microtubules with kinesin across the stream and concentrating them at a horseshoe-shaped collector. Target biomolecule concentrations increase up to three orders of magnitude within one hour of operation.

Switchable pH actuators and 3D integrated salt bridges as new strategies for reconfigurable microfluidic free-flow electrophoretic separation

We present novel strategies for reconfigurable, high-throughput microfluidic free-flow electrophoretic separation using electrically switchable pH actuators and 3D integrated salt bridges to allow rapid formation of stable pH gradients and efficient electrophoresis. The pH actuator is achieved by microfluidic integration of bipolar membranes which change electrolyte pH by injecting excess H(+) or OH(-) ions produced by a field-enhanced water dissociation phenomenon at the membrane junction upon voltage bias. The technique does not require conventional multiple buffer inflows and leaves no gas production as experienced in electrolysis, thus providing stable pH gradients for isoelectric focusing (IEF) separation. With the pH actuator inactivated, the platform can perform zone electrophoretic (ZE) separation in a medium of constant pH. We also describe the use of 3D integrated ion conductive polymers that serve as salt bridges for improving the voltage efficiency of electrophoresis and to allow high throughput. The proof of concept was successfully demonstrated for free-flow IEF and ZE separation of protein mixtures showing the potential and the simplicity of the platform for high-throughput and high-precision sample separation.

High-performance low-temperature poly-Si TFTs crystallized by excimer laser irradiation with recessed-channel structure

High-performance low-temperature poly-Si (LTPS) thin-film transistors (TFTs) have been fabricated by excimer laser crystallization (ELC) with a recessed-channel (RC) structure. The TFTs made by this method possessed large longitudinal grains in the channel regions, therefore, they exhibited better electrical characteristics as compared with the conventional ones. An average field-effect mobility above 300 cm/sup 2//V-s and on/off current ratio higher than 10/sup 9/ were achieved in these RC-structure devices. In addition, since grain growth could be artificially controlled by this method, the device electrical characteristics were less sensitive to laser energy density variation, and therefore the uniformity of device performance could be improved.

Coded-Aperture Imaging Using Photo-Induced Reconfigurable Aperture Arrays for Mapping Terahertz Beams

We report terahertz coded-aperture imaging using photo-induced reconfigurable aperture arrays on a silicon wafer. The coded aperture was implemented using programmable illumination from a commercially available digital light processing projector. At 590 GHz, each of the array element apertures can be optically turned on and off with a modulation depth of 20 dB and a modulation rate of ~ 1.3 kHz. Prototype demonstrations of 4 ×4 coded-aperture imaging using Hadamard coding have been performed. Continuous THz imaging with 8 ×8 pixels has also been demonstrated, using a slowly moving metal strip as the target. In addition, this technique has been successfully applied to mapping THz beams by using a 6 ×6 aperture array at 590 GHz. The imaging results agree closely with theoretical calculations based on Gaussian beam propagation, demonstrating that this technique is promising for realizing real-time and low-cost terahertz cameras for many applications. The reported approach provides a simple but powerful means to visualize THz beams, which is highly desired in quasi-optical system alignment, quantum-cascade laser design and characterization, and THz antenna characterization.

Optical modulation of continuous terahertz waves towards cost-effective reconfigurable quasi-optical terahertz components

We report optical modulation of continuous terahertz (THz) wave in the frequency range of 570-600 GHz using photo-induced reconfigurable patterns on a silicon wafer. The patterns were implemented using programmable illumination from a commercially-available digital light processing (DLP) projector. A modulation depth of 20 dB at 585 GHz has been demonstrated. Modulation speed measurement shows a 3-dB bandwidth of ~1.3 kHz which is primarily limited by the DLP system. A photo-induced polarizer with tunable polarization angle has been demonstrated, showing a 3-dB extinction ratio. Reconfigurable aperture-arrays (4 x 4 pixels) have been attempted for room-temperature coded-aperture imaging using a single Schottky diode detector at 585 GHz. We envision that this technique will provide a simple but powerful means to realize a variety of cost-effective reconfigurable quasi-optical THz circuits and components.

A Nanomembrane-Based Nucleic Acid Sensing Platform for Portable Diagnostics

In this perspective article, we introduce a potentially transformative DNA/RNA detection technology that promises to replace DNA microarray and real-time PCR for field applications. It represents a new microfluidic technology that fully exploits the small spatial dimensions of a biochip and some new phenomena unique to the micro- and nanoscales. More specifically, it satisfies all the requisites for portable on-field applications: fast, small, sensitive, selective, robust, label- and reagent-free, economical to produce, and possibly PCR-free. We discuss the mechanisms behind the technology and introduce some preliminary designs, test results, and prototypes.

Optofluidic sensing from inkjet-printed droplets: the enormous enhancement by evaporation-induced spontaneous flow on photonic crystal biosilica

Novel transducers for detecting an ultra-small volume of an analyte solution play pivotal roles in many applications such as chemical analysis, environmental protection and biomedical diagnosis. Recent advances in optofluidics offer tremendous opportunities for analyzing miniature amounts of samples with high detection sensitivity. In this work, we demonstrate enormous enhancement factors (106-107) of the detection limit for optofluidic analysis from inkjet-printed droplets by evaporation-induced spontaneous flow on photonic crystal biosilica when compared with conventional surface-enhanced Raman scattering (SERS) sensing using the pipette dispensing technology. Our computational fluid dynamics simulation has shown a strong recirculation flow inside the 100 picoliter droplet during the evaporation process due to the thermal Marangoni effect. The combination of the evaporation-induced spontaneous flow in micron-sized droplets and the highly hydrophilic photonic crystal biosilica is capable of providing a strong convection flow to combat the reverse diffusion force, resulting in a higher concentration of the analyte molecules at the diatom surface. In the meanwhile, high density hot-spots provided by the strongly coupled plasmonic nanoparticles with photonic crystal biosilica under a 1.5 μm laser spot are verified by finite-difference time domain simulation, which is crucial for SERS sensing. Using a drop-on-demand inkjet device to dispense multiple 100 picoliter analyte droplets with pinpoint accuracy, we achieved the single molecule detection of Rhodamine 6G and label-free sensing of 4.5 × 10-17 g trinitrotoluene from only 200 nanoliter solution.

Photo-induced spatial modulation of THz waves: opportunities and limitations

Programmable conductive patterns created by photoexcitation of semiconductor substrates using digital light processing (DLP) provides a versatile approach for spatial and temporal modulation of THz waves. The reconfigurable nature of the technology has great potential in implementing several promising THz applications, such as THz beam steering, THz imaging or THz remote sensing, in a simple, cost-effective manner. In this paper, we provide physical insight about how the semiconducting materials, substrate dimension, optical illumination wavelength and illumination size impact the performance of THz modulation, including modulation depth, modulation speed and spatial resolution. The analysis establishes design guidelines for the development of photo-induced THz modulation technology. Evolved from the theoretical analysis, a new mesa array technology composed by a matrix of sub-THz wavelength structures is introduced to maximize both spatial resolution and modulation depth for THz modulation with low-power photoexcitation by prohibiting the lateral diffusion of photogenerated carriers.